1. Field of the Invention
This invention relates to dissipating heat from an enclosed printed wiring board.
2. Description of the Related Art
One common process for definition of signal traces on printed wiring boards is known as a subtractive process. In this process a panel consisting of a copper clad dielectric material is coated with a photoimagable polymer (photoresist). A phototool containing a negative image (clear traces with a black background) of the signal trace layer to be formed is placed over the photoresist coated panel and ultraviolet (UV) light is used to expose and crosslink the photoresist through the clear areas in the phototool, which correspond to the subtractive signal image. Unexposed areas of the photoresist are then removed in a developer solution. The traces are then defined in the copper by exposing the panel to a copper etching solution, where the photoresist acts to protect the copper beneath it while the remaining copper is removed. The photoresist is then removed to complete the process.
A multi-layer printed wiring board typically includes several non-conductive layers of epoxy (a dielectric), several plated-on layers on each side of the dielectric, with spacer layers ("spacers") in between. Traces are etched in each layer to make the various interconnects. One of the many layers of the multi-layer printed wiring board is typically for power while another layer is for ground. The other (non-power and ground) layers are typically thinner and have narrower traces than the power layer or the ground layer.
Increases in semiconductor chip input/output (I/O) lead count in high-performance computers and other systems, and the shrinking size of consumer and automotive electronics are driving the increased wiring density of integrated circuits defined in single and multi-chip packages and printed wiring boards. See Microelectronics Packaging Handbook, 2nd edition, pp. II-63, II-64, and II-117 (1997) by Tummala, Rymaszewski, and Klopfenstein.
To achieve increased wiring density, the signal traces on multi-layer printed wiring boards, and the spaces between the signal traces, are becoming narrower. In such high-density high I/O printed wiring boards, more pin-outs project from each integrated circuit and from all of the integrated circuits than in past practice. On a standard high-density high I/O printed wiring board there can be as many as one hundred integrated circuits, each with fifty to one hundred (or more) leads or pin-outs. As trace widths become narrower, the power density on both the integrated circuits and the printed wiring boards is increasing.
In standard printed wiring boards with standard integrated circuits and standard heat sinks, heat is removed from the board into the interior of a housing. For conventional electronic devices, heat dissipation is commonly performed by permanently attaching a heat sink to an integrated circuit to remove heat generated by operating integrated circuits. Such a heat dissipation technique often results in costly custom modification of the integrated circuit and/or printed wiring board to maximize heat dissipation.
Traditional bosses are ordinarily molded into a plastic housing containing the printed wiring board in order to mount the wiring board.
Flexibility in the design of the electronic device is greatly reduced, and manufacturing costs are increased, if the design of the electronic device or integrated circuit depends on the configuration or characteristics of the housing. As such, there continues to be a need for a heat dissipation technique that does not interfere with design or mounting of the printed wiring boards.